As a light-emitting electronic display device, there is an electroluminescent display (hereinafter abbreviated as ELD). A constituent element of an ELD is, for example, an inorganic electroluminescent element (hereinafter also referred to as an inorganic EL element) or an organic electroluminescent element (hereinafter also referred to as an organic EL element). An inorganic electroluminescent element has been used as a flat-type light source, but high-voltage alternating current is required to drive this light-emitting element.
On the other hand, an organic electroluminescent element has a configuration that a light-emitting layer including a light-emitting compound(s) is provided between an anode and a cathode. An organic electroluminescent element emits light through light emission (luminescence or phosphorescence) upon inactivation of excitons generated by recombining electrons and holes injected into the light-emitting layer. Further, an organic electroluminescent element can emit light with a voltage of several to several dozen volts. Still further, an organic electroluminescent element is a self-light-emitting type and thus achieves rich view angle and high visibility. In addition, because an organic electroluminescent element is a thin film type all-solid element, it is expected for its abilities to save space and to achieve portability.
In addition, an organic electroluminescent element is especially characterized by being a surface light source, different from a practically-used conventional light source such as a light-emitting diode and a cold-cathode tube. As applications effectively utilizing this characteristic, light sources for illumination and backlights of various displays are given as examples. It is also preferable to apply an organic electroluminescent element to a backlight of a liquid crystal full-color display being in significant demand increasing especially in recent years.
When an organic electroluminescent element is used for a light source for illumination such as the above or for a backlight of a display, the organic electroluminescent element is used as a light source emitting white light or light of a so-called light bulb color (hereinafter collectively called white light). Methods for obtaining white light emission in an organic electroluminescent element are exemplified by a method for obtaining white light by mixing colors through using a plurality of light-emitting dopants having different emission wavelengths in a single element, a method for obtaining white light by mixing colors through using light-emitting pixels with different colors such as blue, green and red and making the light-emitting pixels simultaneously emit light, and a method for obtaining white light by using color conversion pigment(s) (for example, a combination of a blue light-emitting material(s) and a color conversion pigment(s)).
However, judging from requirements for a light source for illumination and a backlight such as low cost, high productivity and easy driving method, a method for obtaining white light by mixing colors through using a plurality of light-emitting dopants having different emission wavelengths in a single element is effective for these applications. Thus, research and development regarding this method have been ambitiously conducted.
Giving a detailed description of this method for obtaining white light, a method for obtaining white light by mixing colors of two light-emitting dopants, the colors being complementary color, such as a blue light-emitting dopant and a yellow light-emitting dopant, and a method for obtaining white light by mixing colors of a blue light-emitting dopant, a green light-emitting dopant and a red light-emitting dopant.
For example, a method for obtaining a white light-emitting organic electroluminescence element through doping a blue fluorescent body, a green fluorescent body and a red fluorescent body, all of which are efficient, as light-emitting materials (see Patent Documents 1 and 2, for example).
In addition, in a white light-emitting organic electroluminescence element, there is a method for obtaining emission of light of different colors through, using two or more light-emitting dopants, each of which emits light of color different from each other, in a single layer so as to efficiently transfer energy from a light-emitting dopant with a high emission energy to a light-emitting dopant with a relatively low emission energy, instead of using separate light-emitting layers each emitting light of color different from each other. This method reduces the amounts of light-emitting dopants to be used, and thus is an effective method. For example, Patent Document 3 discloses an organic electroluminescence element characterized by being configured to include an anode, a red light-emitting layer and a blue light-emitting layer in sequence, the red light-emitting layer including at least one green light-emitting dopant.
On the other hand, development of a phosphorescent dopant, which provides an organic electroluminescence element with a higher luminance compared to a fluorescent material (fluorescent dopant), has been vigorously conducted (see Patent Document 4 and Non-Patent Documents 1 and 2, for example), Light emission from a conventional fluorescent material is emission from an excited singlet state. Because the ratio of generating singlet excitons to generating triplet excitons is 1:3, the probability of generation of light-emitting excited species is 25%. In contrast, in the case of a phosphorescent dopant utilizing light emission from an excited triplet state, the maximum internal quantum efficiency is 100% because of the probability of generation of light-emitting excited species and an internal conversion of singlet excitons to triplet excitons. Thus, in principle, a phosphorescent dopant shows efficiency of light emission up to four times as high as that of a fluorescence-emitting dopant.
However, in the case where phosphorescent dopants are used and two or more dopants emitting light of two or more different colors are used in a single layer to emit light of different colors by energy transfer from a dopant having a relatively higher energy level to a dopant having a relatively lower energy level for obtaining a white light-emitting organic electroluminescence element, stability of chromaticity, a length of lifetime and a voltage required for driving according to conditions of driving, a time period of driving and/or a time period of storing are not satisfactory, compared to the case where white light is obtained by stacking a plurality of layers emitting light of colors different from each other. Light sources for illumination are especially demanding for stability of emission color, and therefore ensuring the stability of chromaticity is very important for practical realization of an organic electroluminescence element(s) in light sources for illumination. In addition, dark spots (non-light-emitting spots) are generated resulted from a long-term driving, which requires further technical improvements.
Patent Documents 5, 6 and 7 disclose methods for providing an element not causing pixel defects, requiring low voltage and achieving high efficiency by using a metal complex(es) as a hole-injecting material or a hole-transporting material. However, these documents do not disclose any examples regarding constitutions of a white light-emitting element using a phosphorescent material that emits blue light, and objectives of these documents do not relate to change in chromaticity over time that is important point in use for illumination. Further, these documents do not disclose ionization potentials (hereinafter referred to as IP) of a phosphorescent dopant and a metal complex.